Atomistic model contributes to safety of geosequestration processes

(Nanowerk Spotlight) Global warming, caused by a build-up of greenhouse gases, in particular carbon dioxide, in the atmosphere, has led to numerous proposals on how to capture and store CO2 in order to mitigate the damaging emissions from fossil fuels. In previous Nanowerk Spotlights we looked at various nanotechnology-enabled approaches, ranging from CO2 capture with nanometric thin-film membranes, to the adsorption of greenhouse gases in carbon nanotubes, to a solar-powered process that allows carbon dioxide to be stored as solid carbon or converted in useful products ranging from plastics to synthetic jet fuel.

Today we take a look at carbon sequestration and subsequent storage in geological formations (geosequestration) – a proposal that is already being tested on a large scale. Of the geologic options, sequestering CO2 in coal beds has several advantages. For example, coal can trap CO2 for long periods of time; and potential major coal basins that contain ideal beds for sequestration are near many emitting sources of CO2.

Although nanotechnological approaches, for instance the use of nanoporous materials to separate CO2 from flue gas during post-combustion separation, could help in capturing the gas, it is unlikely they will result in good answers for storing it. The reason is simple: the amount of carbon dioxide that would have to be removed from the atmosphere in order to have an impact on Earth's climate is so vast that it would require massive amounts of these nanostructured materials to be produced and eventually stored somewhere. Currently, this is not feasible from both an economical and a production technology point of view.

The idea behind coal-bed geosequestration is that you inject a huge amount of carbon dioxide into deep unmined coal seams. There are a lot of unmined coal deposits across the planet and several projects are already underway to demonstrate that carbon capture and storage is a technically and environmentally safe way to reduce CO2 in the atmosphere. Take a look at the website of the CO2CRC Otway Project in Australia if you want to learn more about what is going on in this area.

Due to strong adsorption forces, the carbon dioxide will be adsorbed in coal. It will not be desorbed and
gradually transform to solid rocks. Moreover the technology is already developed and in use for oil and gas mining. However, the fundamental problem is so-called adsorption-induced deformation of coal or any other porous material.

"In many previous adsorption studies, scientists have assumed that porous matrices are completely inert," Piotr Kowalczyk tells Nanowerk. "Inert means that you simply load carbon dioxide into very small pores of coal and nothing happens. This, however, is not the case because carbon dioxide produces very high internal stress in pores which, in turn, results in pore deformation. So the carbon matrix is not inert but changes during injection of carbon dioxide at high pressure."

Back in 2008, Kowalczyk, a researcher at the Nanochemistry Research Institute, Department of Chemistry, at Curtin University of Technology in Australia, together with collaborators presented the first atomistic level model for
predicting the adsorption-induced deformation of complex activated carbon ("Adsorption-Induced Deformation of Microporous Carbons: Pore Size Distribution Effect"). In their study, the scientists combined fundamental theory of adsorption, statistical mechanics, and classical elastic theory.

This model is very important for the development of safe and efficient CO2 geosequestration technologies.

Currently, there are at least two reasons to be concerned about adsorption-induced deformation. The first is the adsorption-induced deformation of coal, which can impact the safely of the geosequestration process (keep in mind that millions of tons of CO2 will be injected and there could be catastrophic impacts on the surrounding rock formations if something goes wrong). Here, a fundamental understanding of carbon dioxide deformation of coal is crucial for risk assessment purposes.

The second is the fact that once the coal expands, it is not possible to inject carbon dioxide into it because the transportation channels are getting smaller and the carbon dioxide cannot be transported into coal's pores.

"What experimentalists observed is the lowering of carbon dioxide permeability with time" explains Kowalczyk. "Of course, you can try and use more and more pressure to put carbon dioxide underground, but it is
dangerous because you can completely destroy the coal matrix – the high external pressure will just crush the coal rock. Then the carbon dioxide will be released and potentially poison the groundwater."

He points out that, for researchers developing geosequestration technologies, one fundamental question that needs to be answered is therefore: "Can we really predicted (using computer and fundamental theory) the extend of coal matrix expansion knowing the external operating conditions underground (pressure, temperature, etc.)."

"Yes, we can" is the answer and that is why this new model is so interesting from both a fundamental and
a technological point of view. The presented simulation results can be useful for the interpretation of coal swelling/contraction upon sequestration of carbon dioxide at high pressures and temperatures.

In their simulation, the team studies adsorption-induced deformation of carbonaceous amorphous porous materials due to adsorption of carbon dioxide at 333 K and high pressures.

"We show that adsorbed and compressed carbon dioxide molecules induce very high adsorption stress in the smallest ultramicropores with pore size below 0.31 nm" says Kowalczyk. "At 27 MPa, the solvation pressure in 0.23 nm ultramicropore reaches 3.2 GPa. As pore size increases the solvation pressure is rapidly damped in the range of micropores because thermal fluctuations smooth the packing effects at 333K. Model calculations as well as dilatometric experiment show that any sample of carbonaceous porous solid containing a fraction of the smallest ultramicropores with pore size below 0.31 nm will expand at the studied operating conditions. This is because the adsorbed stress in ultramicropores is much higher in comparison to that in wider micropores."

The team's calculations reproduce quantitatively the strain isotherm of carbon dioxide on carbide-derived activated carbon at 333 K and experimental pressures up to 2.9 MPa.

"Moreover, we extrapolate adsorption and strain isotherms measured by the gravimetric/dilatometric method up to 27 MPa to mimic geosequestration operating conditions" notes Kowalczyk. "And so, we predict that expansion of the studied carbon sample reaches 0.75% at 27 MPa and 333 K."

Snapshots of carbon dioxide confined in selected slit-shaped carbon ultramicropores. The movie shows packing of carbon dioxide in very small carbon pores which can be found in coal. Because of the strong packing in ultramicropores, the internal pressure is very high (orders of GPa). That is why the coal swells.